This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Backbone-backbone hydrogen bonds (H-bond) are common and apparently important features of native protein structures, yet their role in folding transition state structure remains unclear. One possibility is that H-bonds result as a consequence of the formation of native sidechain contacts during folding. Another possibility is that they play a more deciding role by providing a framework that directs the flow of folding toward the native state. To provide a better understanding of the kinetic influence of backbone-backbone H-bonds in protein folding, we investigate the role of helical H-bonding in the folding kinetics of the villin headpiece subdomain (HP-35) using the Phi-value analysis method in conjunction with amide-to-ester (A-to-E) backbone mutagenesis.